F.O.C. stands for FRONT OF CENTER weighting. One of the most often heard complaints from bowhunters is that their broadheads plane off course. If you understand the laws of Physics, regarding motion, you know that an object in motion does not want to change it's direction. So what is the solution? INCREASED MASS IN FRONT OF THE ARROW. The Xtreme series arrowshafts are designed to handle these heavier broadheads and using them will reduce planing. This will also improve your arrows kinetic energy and generally speaking increase the broadheads cutting diameter.